I have no explanation for it, but I also run a tiny mail server and I'm always fascinated that despite extremely low volume I still manage to get through without being flagged or blocked.
Best I can guess is that my host's netblock just happens to be sparkling clean, but it sounds like even that may not be enough anymore
Similar story here; my only guess (which I don't want to verify in case it jinxes it) is that I've been on the same name and netblock for an extremely long time (~20 years) and so I'm grandfathered in to a lot of undocumented IP rules at the big houses. Long may it continue.
Best I can guess is that my host's netblock just happens to be sparkling clean, but it sounds like even that may not be enough anymore